Polymeric rear projection screens

ABSTRACT

Rear projection screens that consist essentially of polypropylene can be improved significantly by incorporating into the screen one or more &#39;&#39;&#39;&#39;crystal deforming&#39;&#39;&#39;&#39; additives, such as finely divided starch, sucrose octoacetate (and) cellulose acetate butyrate, and calcium carbonate which additives (a) are essentially non-nucleating in their effect on the crystal structure of the screen material, (b) are able to deform the spherulites in the crystal structure to thereby increase light diffusion, and (c) have refractive indices within + OR - 0.05 of that of the polypropylene in the screens.

United States Patent 11 1 Lu et al.

11] E Re. 28,516

[ Reissued Aug. 12, 1975 POLYMERIC REAR PROJECTION SCREENS [22] Filed: Mar. 29, 1973 21 Appl. No.: 345,921

Related U.S. Patent Documents 3,367,926 2/l968 Voeks 260/949 X OTHER PUBLICATIONS Kuhre, C. J et al. Crystallization-Modified Polypropylene, SPE Journal, Oct. 1964, pp. 1 l l3l l 19.

Primary ExaminerMayer Weinblatt Assistant ExaminerEdith R. Buffalow Attorney, Agent, or Firm-E. W. Milan Reissue of: [57] ABSTRACT [64] Patent No.: 3,573,141 D i Issued; Mar. 30 19 Rear pro ection screens that consist essentially of APPL 838,304 polypropylene can be improved significantly by incor- Filed; July 1, 1969 porating into the screen one or more crystal deform- Y ing additives, such as finely divided starch, sucrose '[52]-- U.S. Cl. 428/213; 428/323; 428/326; Octoacetate cellulose, acetate butyrate, and 428/332 calcium carbonate which additives (a) are essentially 51 Int. Cl. c031: 21/60; C081 45/14 non-nucleating in their effect on the crystal Structure [58] Field of Search 428/323, 326, 332; of the Screen material, (b) are able to deform the 260/4] C, 949 F, 96 350/127 162 spherulites in the crystal structure to thereby increase I light diffusion, and .(c) have refractive indices within 56] References cited 0.05 of that of the polypropylene in the screens. UNITED STATES PATENTS 19 Claims, 1 Drawing Figure 2,480,031 8/l949 Kellogg l6l/3.5

a k v I I 'I x I ,7 x x Y I Q 16 Reissuecl Aug. 12, 1975 Re. 28,5 v

POLYMERIC REAR PROJECTION SCREENS Matter enclosed in heavy brackets I: 1 appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

The present invention relates to polymeric rear projection screens in which the polymeric portion of the screens consist essentially of polypropylene. The screens additionally contain,' dispersed therethrough, one or more crystal deforming agents, which have the peculiar ability to improve that property of the screens relating to hot spot without detracting substantially from the other excellent optical properties of the screens.

in copending US. patent application Ser. No. 838,528 filed July 1, 1969, special processes for manufacturing rear projection screens made from molten polypropylene are described in detail. Also described in detail in said copending patent application are the resulting valuable screen materials. These polypropylene screen materials perform under actual use conditions in a manner that is considered excellent. Apparently the valuable optical properties of such screen ma terials are due to the unique arrangement (and relative proportions) of regions of crystalline and relatively .amorphous polypropylene in the sheets, as well as the sizes and types of crystals involved in those regions. Thus, in such sheets, there is a central core of almost totally crystallized polypropylene (which core" is at least three-fourths of the thickness of the sheets). On either side of this core there is a layer of apparently relatively less crystallized polypropylene (in a sandwich-like arrangement) with the less crystallized layers being at the surfaces of the sheets (which sheets in turn are from about 20 to about 80 mils thick, and preferably are from about 25 to about 60 mils thick). The en tire screen, however, is composed of at least about 60 weight percent (preferably from about 70 to about 90%) of spherulite structures having average diameters (measured at their widest point) of from about to about 20 microns. The larger spherulite crystals in the core" of such valuable. polypropylene rear projection screen materials have diameters of from about l5 to about 45 microns (preferably from about 20 to about 40 microns) at their widest point. The largest crystals in the outer, less crystalline sandwich layers at the sheet surfaces are almost invariably smaller than mi crons in diameter. The processes for manufacturing such sheets entails, briefly, (a) casting or extruding a melt" consisting essentially of polypropylene in the form of the fairly thick sheet onto a heat transfer surface maintained at a temperature which is low enough to cause only the surface layer( s) of the sheet to harden sufficiently to maintain the form integrity of the sheet (generally at a heat transfer surface temperature of from about 85C. to about l70 C. and (b) thereafter cooling the sheet in such a way (in an area in which the surfaces of the sheet are maintained at from about 85 C. to about 140 C.) to form the desired, largely crystalline core having the requisite spherulite sizes, and distribution.

The evaluation of rear projection screens can be described in toms of four separate, distinct optically appreciative elements; namely, in terms of l contrast," (2) brightness," (3) scintillation and (4) hot spot. An ideal rear projection screen has practically no hot spot" (an area on the screen which is dis' tinctly brighter than the remainder of the screen), practically no scintillation (the appearance of tiny, spar kling spots over the surface of the screen), maximum contrast and high brightness. Of these elements, the elimination ofhot spot and scintillation are of perhaps primary importance, because these tend to induce eye fatigue with any long period of continual viewing. One method for determining the relative merits and the relative acceptability of a given rear projection screen material is to separately evaluate the material for each of these four major elements in a somewhat subjective manner as follows:

A method by which the relative merits and acceptability ofa given rear projection screen material can be evaluated for each of the aforementioned characteristics in a somewhat subjective manner is to assign a progressive numerical value for each of the characteristics. For example, ifa value of 0-9 is assigned to each characteristic with a rating of 0 being indicative of the very poorest quality and a rating of 9 being indicative of the best possible quality, then the evaluated numerical rating for each of the four characteristics can be added together to determine a particular quality or Q rating for any given screen material. Such evaluations are as signed when a material is tested under actual conditions by projecting an image on the screen in a conven tional manner. By this method a theoretically perfect screen would have a Q rating of 36 and a screen of inferior quality would have a correspondingly lower Q rating. This method, as described above, is not all inelusive because it relies on an individuals visual analysis which can and will vary from person to person. However, the method does provide a way of evaluating the characteristics of different screen materials so that at least a subjective comparison can be made. A more practical Q rating might be obtained, if the same evaluations were made by several persons and then averagedv A conventional rear projection screen (A) comprising a glass plate coated with a wax diffusing layer has a Q rating of 24 which was determined as follows:

SCREEN A Hot spot 9 Scintillation 2 Contrast g Brightness 5 0 rating 24 Hot Scintil- Screen spot lation Contrast Brightness 0 rating By comparison, a rear projection screen of the invention of said copending US. patent application Ser. No. 838,528 was evaluated as follows:

From the Q ratings and individual element evaluations set out above for the conventional screens (A, B and C), it can be appreciated that in each instance, the commercially available rear projection screen had at least one major shortcoming, which, in fact, detracted significantly from its overall desirability as a rear projection screen. These are typical data for conventional rear projection screens.

Although such polypropylene screens as those de scribed and claimed in said copending patent application have significantly improved properties, as compared with conventional rear projection screens, it has now been discovered that still better properties can be imparted to the polypropylene rear projection screen 7 materials, provided that at least about 2 (preferably from about 4 to about 15, and usually at most about weight percent, based on the weight of the resulting screen material, of'one'or more special additive which ,will herein sometimes be referred to as a crystal deforming material (in finely divided form) is also present, uniformly dispersed through the sheet of rear projection screen material. The special additives that are useful in the successful practice of this invention are called crystal deforming materials or agents because of what is believed to be the mechanism whereby the major beneficial effect upon the ultimate performance of the rear projection screens of this invention (the substantial elimination of the undesired hot spot" effect described hereinbefore) is accomplished."

' small spherulites and relatively clear sheet. Slow cooling produces large spherulites and translucent sheet. Copending US. patent application 838,528, filed concurrently herewith, describes the method of producing rear projection screens to form a heterogeneous structure'having spherulites of varying sizes and comprised of at leasttwo or more types of the four type of polypropylene spherulites. (A review of crystal morphology of polypropylene isgiven in Polymer Single Crystals" by P. H. Geil, lntersciencc Publishers, 1963.) The valuable optical characteristics of these screens are believed to be due to an almost totally crystalline central core" (composed of the aforementioned heterogeneous structure having spherulites of varying types and sizes) which core" comprises at least three fourths of the thickness of the sheets.

' material).

In this present application crystal deforming agents" are added to increase even further the heterogeneous structure formed by slow cooling. Photomicrographs reveal that these agents are not incorporated into the growing spherulite itself (such as nucleating agents), but are pushed aside as the crystalline structure develops until finally they are squeezed between spherulites and deform the normally spherical shape of the spherulite. These deformed spherulites have increased light diffusion capabilities. For rear projection use this effect is very desirable for it reduces hot spot" (an area on the screen which is distinctly brighter than the remainder of the screen) and also permits use of a thinner screen. By reducing screen thickness more light transmission results which increases brightness and usually contrast as well.

Materials that can be used successfully as crystal deforming materials" in the practice of this invention are those powdered materials (a) having particles substantially within the range of from about 5 to about microns (wherein the average diameter of said particles is from about 10 to about 30 microns), (b) having a refractive index within about 0.05 unit of that of the polypropylene in the rear projection'screens, and (c) having the capability of being present in the molten sheet While it is being solidified (and while the crystals therein are being formed and growing) without causing a significant amount of additional crystal nucleation (as compared to that which ordinarily occurs under similar conditions, but in the absence of the crystal deforming Materials having this latter capability (which materials will herein sometimes be referred to as non-nucleating materials) can readily be identified by simply preparing a 10 weight percent dispersion of the particular material in molten propylene, casting on a steel plate and pressing it into a 30 mil layer of the molten mixture,'and permitting the resulting layer to solidify under ambient room temperature conditions. Layers that contain non-nucleating materials de velop a highly opaque appearance, whereas those that cause an undesirable degree of nucleation cause the polypropylene films to appear almost clear when they are cooled andproduce severe hot spot. (The nucleation induces the formation of many crystals of very small size which are unable to produce the high degree of light diffusion necessary for rear projection screen use.) An example of a finely divided material having the requisite refractive index (detailed above), but

which does not qualify as a crystal deforming material" useful in the practice of this invention is finely divided silica. This material is a powerful nucleating agent, causing the formation of an extremely clear polypropylene sheet (which therefore is not acceptable as a rear projection screen) when it is tested as a potential crystal deforming material in this context.

Examples of crystal deforming materials" that are utilized in the formation of some of the preferred embodiments of this invention are starch, sucrose octoacetate and cellulose acetate butyrate. Screens or sheets consisting essentially of (a) polypropylene I: I with (b) one or more ufsat'd crystal deforming materi als [plus] and/or (0) powdered calcium carbonate constitute still another group of preferred embodi ments of this invention. The powdered calcium carlxmare employed in .ruclt embtxlimems inherently meets the criteria described hereinbefure for crystal deforming materials" and, therefore, can be used as the sole crystal Re. 28 ,5 l 6 deforming material" as well as in combination with other crystal deforming materials" in practicing the in ven tion.

EXAMPLE 1 Eleven parts of -25 micron average particle size wheat starch powder (having a refractive index of 1.50) are prepared for admixture with crystallizable polypropylene by first blending the starch with an equal volume of xylene in which 033 part of amorphous polypropylene is dissolved, and then removing the xyleneby a slow evaporation procedure. The resulting coated starch particles are then dry blended with 100 parts of polypropylene the polypropylene having a melt viscosity of 2.5, a density of 0.90 gram/co, and a refractive index of 1.49). The resulting blend is then melted and extruded at a temperature of 200 C. directly onto the surface of a steel roll, which surface is maintained at 90 C. The thickness of the sheet is 30 mils. The temperature of theair surrounding the roll is maintained at 93 C. The diameter of the cool roll is 2.5 inches, and the sheet is cast continuously at a rate of 3 ft. per minute. The total amount of time during which the extruded sheet is maintained in contact with the surface of the cool roll is 9 seconds, this time being sufficient to cause the surface of the sheet to set up,"

thereby forming thin layers of polypropylene of low crystallinity at both surfaces of the sheet. The sheet is .then passed onto the surface of a second roll of the same type and maintained under the same conditions. Upon leaving the surface of the second of these relatively cool rolls the sheet is dropped into'a vertical chamber in which the air temperature is maintained at 93 C. The surface of the sheet and the air surrounding the sheet are maintained at about this temperature for a total of 90 seconds by passing the sheet before a series of electric heaters mounted along the rolls of the chamber. During the period of time in which the sheet is passed through the vertical chamber, an extensive translucency isobserved developing in the sheet. This results from the formation of a multiplicity of the relatively larger crystals of the necessary size in the core of the sheet. The sheet is then removed from the chamber and cooled under ambient conditions to room temperatures, at which point it is cut into individual sheets, which in turn are pressed in a conventional hydraulic press under a pressure of 140 psi. between one very smooth steel plate and one steel model plate having the desired roughened surface of microinches. The resulting sheet yields a Q'Rating of 33. When tested in a conventional rear projection apparatus, in which microfiches recorded on a sheet of colored transparent film are used, it is observed that this screen transmits a very sharp image which is not only very free of undesirable haze but is also a true rendition of the colored material on the original microfiche.

It has been found that the polypropylene that can be used successfully in the practice of this invention are those crystallizable polypropylenes having a melt flow rate or viscosity (measured in dg./min. at 230C. under a 2.!6 kg. load; A.S.T.M. standard test D1238) of below about 90, preferably between about 1.5 and about 10. Other materials that do not detract substantially from the crystallite distribution and sizes described hereinbefore can also be present in the scree materials of this invention. V

EXAMPLE 2 A screen is prepared as in Example 1, except that 5 parts of very finely powdered 10-40 micron diameter particles) sucrose octoacetate are used in place of the starch.

EXAMPLE 3 Hot Scintil- Example spot lation Contrast Brightness 0 rating As it was pointed out in the eopending patent application described above, and as set out in the latter part of the above example, it is sometimes desirable to have one of the surfaces of rear projection screens uniformly and randomlyroughened to the extent of from about l0 to about 30 microinches, and preferably from about I 15 to 25 micro'inches (as measured by a commercial surface roughness analyzer such as the Bendix Prof l corder The roughness should be random in nature such as that obtained by sandblasting, etching, and the like. The roughened surface can be applied to the rear 7 projection screens of this invention by any of a number of ways, including that set out in Example I. Also, when the screens are to be used'to read negative materials for any extended period of time, it is preferable to view the negative materials through a relatively darker, neutral density colored layer. Thus, in a special adaptation of this invention, sufficient'neutral density dye (or combination of dyes) can be either added tothe polypropylene (or coated on the screen material) in amounts sufficient to produce an increase in diffuse density in the resulting composition of as much as 0.1 or'more (but preferably at most about 0.2). Alternatively, athin, transparent, neutral density dyed polymeric film can be utilized in combination with the special screen material described above, if desired. (In this instance, the surface of the screen material facing the person using the viewer neednot be roughened, but' one surface of the neutral density film material can be roughened).

Consequently, a particularly preferred embodiment of the present invention relates to a polymeric article comprising a polypropylene screen layer or sheet as'detailed hereinbefore [which consists essentially of the.

mixture of crystallized polypropylene and crystal deforming agents(s)],-and "a neutral color" density dyed polymeric film having a diffuse density of at most about 0.2, one outer surface of said article having the specially roughened surface described above.

Following a procedure such'as that of Example 1. above, several other materials have been evaluated as being possibly useful as crystal deforming materials" in polypropylene rear projection screens with varying poor effects. None of the materials, however, proved acceptable for this use. Materials tested without success included silica, titanium dioxide, talc, magnesium silicate, clay and carbon black.

Referring now to the accompanying drawing;

The FIGURE is a schematic view showing the arrangement of a rear projection screen relative to a pro jector and a viewer.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications will be effected within the spirit and scope of the invention.

What is claimed is:

1. ln a polymeric sheet especially adapted for use as a rear projection screen; said sheet (a) consisting essentially of polypropylene having a melt flow rate of at most about 90, (b) having a thickness [of from about 20 to about 80 mils] such that it transmits light; and having two surface layers containing relatively smaller crystalline spherulites and a core layer containing relatively larger crystalline spherulites; the diameter of the larger crystals in said core layer being between about 15 and about 45 microns and said core layer being at least three-fourths as thick as said sheet, and said sheet containing at least about 60 weight percent of crystalline spherulites having average diameters of from about to about 20 microns; the improvement which comprises incorporating evenly through said sheet at least about 2 weight percent of finely divided, particulate crystal deforming material," said crystal deforming material (i) being non-nucleating, (ii) having a refractive index within about 0.05 unit of that of said polypropylene, and (iii) having particle diameters in the range of from about 5 to about 50 microns.

2. An improved rear projection screen material as in claim I, wherein said crystal deforming material is starch.

3. An improved rear projection screen material as in claim 1, wherein said crystal deforming material" is sucrose octoacetate.

4. An improved rear projection screen material as in claim 1, wherein said crystal deforming material is cellulose acetate butyrate.

5. An improved rear projection screen material as in claim 1, wherein said polymeric sheet has one randomly roughened surface; the roughness of said roughened surface being about to about 25 microinches.

6. An improved rear projection screen material as in claim [I] 2. wherein said screen [also] contains from about l to about [0 weight percent of finely divided calcium carbonate as a crystal deforming material.

7. A polymeric sheet consisting essentially of polypropylene having a melt flow rate between about 1.5 and about 10 and at least one crystal deforming material" said sheet (a) having a thickness [of from about 20] up to about mils and (b) having two surface layers containing relatively smaller crystalline spherulites and a core layer containing relatively larger crystalline spherulites; the diameter of the larger crystals in said core layer being from about l5 to about 45 microns, measured at their widest point, said sheet containing at least about 60 weight percent of crystalline spherulites having average diameters of from about 5 to about 20 microns, and said core layer being at least three-fourths of the thickness of said sheet, and said crystal deforming material (a) being present in said sheet in an amount equal to from about 4 to about 15 weight percent of said sheet;

(b) having a refractive index within about 0.05 unit of that of said polypropylene;

(c) consisting essentially of particles having maximum diameters of from about 5 to about 50 microns; and

(d) being non-nucleating with respect to the crystallization of said polypropylene.

8. A polymeric sheet as in claim 7, wherein said crystal deforming material is starch.

9. A polymeric sheet as in claim 7, wherein said crystal deforming material" is sucrose octoacetate.

10. A polymeric sheet as in claim 7, wherein said crystal deforming material" is cellulose acetate butyrate.

11. A polymeric sheet as in claim 7, wherein said sheet has one randomly roughened surface; the roughness of said roughened surface being from about 10 to about 25 microinches.

12. A polymeric sheet as in claim 11, wherein said crystal deforming material is starch.

13. A polymeric sheet as in claim 11, wherein said crystal deforming material is sucrose octoacetate.

14. A polymeric sheet as in claim 11, wherein said crystal deforming material" is cellulose acetate butyrate.

15. A multi-layer article comprising (a) a polymeric sheet as in claim 7, and

(b) a neutral color density polymeric film having a diffuse density of at most about 0.2;

one outer surface of said article being randomly roughened and having a roughness of from about 10 to about 25 microinches.

16. An improved rear projection screen material as in claim I, wherein said crystal deforming material" is calcium carbonate.

17. A polymeric sheet as in claim 7, wherein said crystal deforming material is calcium carbonate.

[8. An improved rear projection screen material as in claim 1 wherein the sheet has a thickness of from about 20m about 80 mils.

19. A polymeric sheet as in claim 7, wherein the sheet has a thickness of from about 20 to about 80 mils. 

1. IN A POLYMERIC SHEET ESPECIALLY ADAPTED FOR USE AS A REAR PROJECTION SCREEN, SAID SHEET (A) CONSISTING ESSENTIALLY OF POLYPROPYLENE HAVING A MELT FLOW RATE OF AT MOST ABOUT 90, (B) HAVING A THICKNESS ( OF FROM ABOUT 20 TO ABOUT 80 MILS ) SUCH THAT IT TRANSMITS LIGHT, AND (C) HAVING TWO SURFACES LAYERS CONTAINING RELATIVELY SMALLER CRYSTALLINE SPHERULITES AND A CORE LAYER CONTAINING RELATIVELY LARGER CRYSTALLINE SPHERULITES, THE DIAMETER OF THE LARGER CRYSTALS IN SAID CORE LAYER BEING BETWEEN ABOUT 15 AND ABOUT 45 MICRONS AND SAID CORE LAYER BEING AT LEAST THREE-FOURTHS AS THICK AS SAID SHEET, AND SAID SHEET CONTAINING AT LEAST ABOUT 60 WEIGHT PERCENT OF CRYSTALLINE SPHERULITES HAVING AVERAGE DIAMETERS OF FROM ABOUT 5 TO ABOUT 20 MICRONS, THE IMPROVEMENT WHICH COMPRISES INCORPORATING EVENLY THROUGH SAID SHEET AT LEAST ABOUT 2 WEIGHT PERCENT OF FINELY DIVIDED, PARTICULATE "CRYSTAL DEFORMING MATERIAL", SAID CRYSTAL DEFORMING MATERIAL (1) BEING NON-NUCLEATING, (II) HAVING A REFRACTIVE INDEX WITHIN ABOUT 0.05 UNIT OF THAT OF SAID POLYPROPYLENE, AND (III) HAVING PARTICLE DIAMETERS IN THE RANGE OF FROM ABOUT 5 TO ABOUT 50 MICRONS.
 2. An improved rear projection screen material as in claim 1, wherein said ''''crystal deforming material'''' is starch.
 3. An imProved rear projection screen material as in claim 1, wherein said ''''crystal deforming material'''' is sucrose octoacetate.
 4. An improved rear projection screen material as in claim 1, wherein said ''''crystal deforming material'''' is cellulose acetate butyrate.
 5. An improved rear projection screen material as in claim 1, wherein said polymeric sheet has one randomly roughened surface; the roughness of said roughened surface being about 10 to about 25 microinches.
 6. An improved rear projection screen material as in claim (1) 2 , wherein said screen (also) contains from about 1 to about 10 weight percent of finely divided calcium carbonate as a ''''crystal deforming material.''''
 7. A polymeric sheet consisting essentially of polypropylene having a melt flow rate between about 1.5 and about 10 and at least one ''''crystal deforming material'''' said sheet (a) having a thickness (of from about 20) up to about 80 mils and (b) having two surface layers containing relatively smaller crystalline spherulites and a core layer containing relatively larger crystalline spherulites; the diameter of the larger crystals in said core layer being from about 15 to about 45 microns, measured at their widest point, said sheet containing at least about 60 weight percent of crystalline spherulites having average diameters of from about 5 to about 20 microns, and said core layer being at least three-fourths of the thickness of said sheet, and said ''''crystal deforming material'''' (a) being present in said sheet in an amount equal to from about 4 to about 15 weight percent of said sheet; (b) having a refractive index within about 0.05 unit of that of said polypropylene; (c) consisting essentially of particles having maximum diameters of from about 5 to about 50 microns; and (d) being non-nucleating with respect to the crystallization of said polypropylene.
 8. A polymeric sheet as in claim 7, wherein said ''''crystal deforming material'''' is starch.
 9. A polymeric sheet as in claim 7, wherein said ''''crystal deforming material'''' is sucrose octoacetate.
 10. A polymeric sheet as in claim 7, wherein said ''''crystal deforming material'''' is cellulose acetate butyrate.
 11. A polymeric sheet as in claim 7, wherein said sheet has one randomly roughened surface; the roughness of said roughened surface being from about 10 to about 25 microinches.
 12. A polymeric sheet as in claim 11, wherein said ''''crystal deforming material'''' is starch.
 13. A polymeric sheet as in claim 11, wherein said ''''crystal deforming material'''' is sucrose octoacetate.
 14. A polymeric sheet as in claim 11, wherein said ''''crystal deforming material'''' is cellulose acetate butyrate.
 15. A multi-layer article comprising (a) a polymeric sheet as in claim 7, and (b) a neutral color density polymeric film having a diffuse density of at most about 0.2; one outer surface of said article being randomly roughened and having a roughness of from about 10 to about 25 microinches.
 16. AN IMPROVED REAR PROJECTION SCREEN MATERIAL AS IN CLAIM 1, WHEREIN SAID "CRYSTAL DEFORMING MATERIAL" IS CALCIUM CARBONATE.
 17. A polymeric sheet as in claim 7, wherein said ''''crystal deforming material'''' is calcium carbonate.
 18. An improved rear projection screen material as in claim 1 wherein the sheet has a thickness of from about 20 to about 80 mils.
 19. A polymeric sheet as in claim 7, wherein the sheet has a thickness of from about 20 to about 80 mils. 